Method of treating aluminum base alloys which are subject to growth



Patented Sept. 15, 1931 UNITED STATES PATENT orl-"lca v ROBERT E. ARCHER AND WILLIAM L. FINE, OF CLEVELAND, OHK), ASSIGNORS TO ALUMINUM COMPANY OF AMERICA, OF PITTSBURGH, PENNSYLVANIA, A CORPO- RATION OI PENNSYLVANIA METHOD OF TREATING ALUMINUM BASE ALLOYS WHICH ARE SUBJECT TO GROWTH No Drawing.

The invention relates to the treatment of aluminum base alloys WhlCh undergo permanent increases in volume when heated.

While particularly applicable to the treat-- ment of pistons and other aluminum base alloy articles which are used as parts either moving or fixed, of internal combustion engines and the like, in its broader aspects the invention applies to the treatment of all k nds of articles used under various -cond1t1ons where increases in their volumes are undesirable, and which are formed of aluminum base alloys of the type explained.

It is well known that some aluminum base alloys, when subjected to elevated or even slightly elevated temperatures, undergo a gradual and permanent increase 1n volume which finally attains a maximum, while other such alloys are not so characterized even when held at elevated temperatures for an extended period of time. At present, actual experiment is necessary to determine whether a particular aluminum base alloy w1ll or will not undergo this permanent increase in volume, there being no reliable'means of predicting it. This increase in volume is different in nature from the thermal expansion of a metal in which an equal contraction occurs on cooling, and the phenomenon has been designated as growt or permanent growth, and is so designated herein.

The fact that some aluminum base alloys grow when subjected to elevated temperatures, by which is meant here and throughout this specification temperatures notgreatly exceeding 350 C. has long been recognized as a disadvantage which in many cases should be overcome before the alloys are satisfactory for uses where their retention of original dimensions is an important factor. To preclude growth in finished articles, thermal methods have been devised by which growth is fully developed before the articles are finally shaped to finished dimensions. Usually the method consists in heating the alloys to a temperature of from about 27 5 to 450 C. for a period of time, customarily varying between one to ten hours, or until maximum growth is obtained. To the best of our Application filed December 22, 1828. Serial No. 328,048.

knowledge this is the only method which has been commercially used for this purpose.

Because of the relatively high temperatures used, this method has the serious disadvantage of softening the alloys because they lose a substantial portion of their hardness when heated for considerable periods of time at the above mentioned temperatures, within the stated range of which is usually found the commercial annealing temperature of the particular alloy being treated, or in other words the temperature at which the alloy can be most efficiently softened. Consequently, aluminum base alloys in which maximum volume changes have been developed by thermal treatments from about 27 5 to 450 C. have usually lost substantially proportions of their original or attainable hardness.

Satisfactory methods of developing maximum growth in aluminum base alloys are of considerable. importance for many uses of the articles of which they are formed, but in no field of industrial application has the necessity of developing maximum growth as yet become so important as in the case of aluminum base alloy pistons. The conditions of service of such pistons make the hardness of the alloys, and their ability to maintain required dimensions, properties of great importance; and the substantial decrease in hardness caused by the prevailing practice in heating the alloys to obtain maximum growth has considerably detracted from the peculiar usefulness of the alloys as materials for the manufacture of pistons.

The object of the invention is to provide a method of thermal treatmentof aluminum base alloys which grow, whereby maximum growth of the alloys may be effected without a simultaneous substantial decrease in hardness, and which method is simple, effective and practical for treating aluminum base alloy pistons and other articles in which hardness is important.

. 'The'invention is predicated upon our disabout 150 to 265 C. and continuing the heating until the hardness of each alloy has assed through its maximum and the alloy has attained a substantially complete and permanent increase in volume. Although it has heretofore been considered that these temperatures are too low for such treatment, we have found that, after adequate treatment at them, aluminum base alloys do not undergo further growth in service.

In the practice of the invention, the selection of a particular treating temperature depends upon the aluminum base alloy to be treated and its desired final hardness. In general, the lower temperatures of treatment produce higher hardnesses at maximum growth. Within the temperature range of about 150 to 265 C. a considerably higher hardness is attained at the point of maximum growth than obtained by the prior processes, and, when maximum hardness is desired, it is preferred to use a temperature of from 150 to 185 C. As to time, that required to obtain maximum growth depends to a large extent upon the temperature used, and to a lesser extent upon the aluminum base alloy to be treated. The nature of one such alloy as compared with another may be such as require considerable variation of the period of heating at any given temperature, but for any one aluminum base alloy the time of heating necessary to develop maximum growth is shorter the higher the temperature, as will presently appear from examples which will be given.

To further an understanding of our inventionit seems desirable to first explain the manner in which certain well known heat treatable aluminum base alloys respond to heat treatment to produce structural changes in them. Such heat treatment usually comprises three steps, namely, a solution heat treatment, a rapid cooling or quenching to ordinary room temperature, and an aging treatment. The solution heat treatment consists in heating a so-called heat-treatable alloy to a temperature slightly below the eutectic temperature, or the temperature of incipient fusion of the alloy, for a period of time suflicient to place a substantial part of the undissolved soluble alloying constituents in solid solution with the aluminum. The second or cooling step consists in rapidly cooling the alloy from its solution heat treatment to retain at ordinary temperature the solid solutions formed during the solution treatment. The third or aging treatment causes the unstable solid solutions thus retained to break down in such a manner that the hardness and tensile strength of the alloy is increased, which increase in hardness and strength is probably caused by the precipitation of dissolved constituents from the solid solution. The aging treatment is usually effected by heating the alloy to a relatively low temperature for such period of time as is necessary to develo the most favorable combination of har ness and strength, and in practice this time does not exceed the point at which the strength of the alloy has become maximum and its hardness has approached a maximum. Continuation of ageing causes the strength and hardness to pass through their respective maxima, and to subsequently decrease, over-aging having been considered undesirable because it produces less than maximum strength and hardness.

In working with heat treatable aluminum base alloys we have discovered that their solution heat treatments decrease their volumes as compared with those as cast and that the aging treatments, and also thermal treatments at considerably higher temperature than the aging treatments, increase their volumes as compared to those as cast. In the light of what is known and believed concernng the structural changes which take place 1n aluminum base alloys during heat treatments, we have come to the conclusion that their volume changes are caused by either the solution, in the case of decreased volume, or the precipitation, in the case of increased volume, of the soluble alloying constituents. Accordingl the henomenon of growth in aluminum ase al oys may be attributed to the difference in volume between an alloying constituent as it is present in solid solution in aluminum, and as it is present in the undissolved or precipitated condition. Thus, an aluminum base alloy which has been submitted to a solution heat treatment, or is in the as cast and untreated condition, contains a certain amount of alloying constituent held in a solid solution with the aluminum, which, when the alloy is subjected to certain elevated temperatures, tends to precipitate from the solid solution in a form which in volume is larger than the volume of the same constituent in solid solution, causing the alloy to increase in Volume, or, in other words, to grow. The magnitude of this increase in volume is affected by the size and distribution of the precipitated particles. While we believe this explanation to be correct, it will be understood that the theory is merely explanatory, and that it is not limiting upon the scope of the appended claims.

As above mentioned, we have discovered the heretofore unrecognized fact that maximum growth can be developed at materially lower temperatures than heretofore used without the substantial loss in hardness which attends the prior growth-developing treatment. We have also discovered that by these low temperature treatments full growth cannot be obtained unless the treatment is extended beyond the point where maximum hardness and strength is developed, with respect to which our process differs in substance from the aging treatments which have been .Brinell hardness then being and are bein used for the development of strength an hardness in heat treatable aluminum base alloys, and which have been practically limited to temperatures of from about 100 to 200 C. to attain their intended purposes. The method herein provided, and which as stated may be practiced with good results at temperatures from about 150 to 265 C. contemplates the develo ment of maximum growth in the allo s, an is therefore continued beyond the point of maximum hardness to the pointof maximum growth. Although in obtainin maximum growth the hardness of an alloy 1s developed to a maximum and then decreased, the hardness finally retained by the alloy is much greater than the hardness of the alloy after being subjected to the prior growth-developing treatments at temperatures from about 275 to 450 C. and the alloy is sufiiciently hard for the service conditionsof pistons or other articles where both hardness and the elimination of changes in volume are factors of prime importance.

By way of example, the advantages of this invention over the prior practice are illustrated by the results produced on two chill cast pistons formed of almunium base alloys containing 10% copper as the main alloying constituent, each of which pistons was given a solution heat treatment at 495 C. and quenched in water. One of these pistons was then subjected to a growth-developing treatment consisting of heatin it at 375 C. untilmaximum growth was 0 tained, after which thepiston had a Brinell hardness of 61 and a unit growth per inch of 0.00022, of an inch. The other piston was subjected to a growth-developing treatment according tothis invention by heating it at a temperature of 225 C. until fullgrowth was obtained, after which the piston had a Brinell hardness of 121 and a unit growth per inchof 0.00091 of an inch.

As examples of the various temperatures that maybe used in, and the various times required for, practicing the growth-developing treatment herein provided, a chill cast aluminum-copper alloy piston required a heating period of 200 hours at 154 C. before maximum growth was obtained, the Brinell hardness of the piston then being 142. Another chill cast piston made from the same alloy required only 86 hours at 170 C. to develop maximum growth, its Brinell hardness then being 142, while a third chill casting of the same alloy required 19 hours heating at 225 Cfto develop maximum growth, its 120; In all of these cases a growth per inch of 0.0006 of an inch was obtained, and with further heating no additional measurable growth took'place.

It is apparent from these examples that, because of the short heating period, a treatment at 225 C. is better adapted to commercial practice than, for example, a treatment at 154, 0. However,when as a governing factor the highest obtainable hardness is requlred at the point of maximum. growth, the longer heat treatments at lower temperatures may be economically feasible. The hardness developed in each of the foregoing examples 1s considerably higher than that developed by the prior treatments, and the Brinell hardness of 120 produced by a treatment for 19 hours at 225 C. is sufliciently high for most purposes.

As further examples of the practice of our invention, a chill cast aluminum base alloy containing 7% copper, 7% silicon, 7 5% mckel and 0.58% iron, obtained its maximum growth after being heated for 22 hours at 225 C. Its Brinell hardness was then 114, 'and 1t had a unit growth per inch of 0.00091 of an inch. Another aluminum base alloy containing 14% silicon 2% nickel, 1% magnesium and 0.75% copper cast in the form of a piston was given a solution heat treatment and subsequentlyheated for 17 hours at 225. C. at which time its maximum growth was obtained. The piston then had a Brinell hardness of 107 and a unit growth per inch of 0.00028 of an inch. Still another aluminum base alloy containing 4% copper and 2% nickel cast in the form of a piston was given a solution heat treatment at 535 C. for 16 hours and quenched in'water, after which it had a Brinell hardness of 60. This piston was then heated for 36 hours at 225 C. when its full growth was obtained, and it then had a Brinell hardness of 80 and a unit growth per inch of 0.00075 of an inch.

These examples illustrate the applicability of our invention to various commercial aluminum base alloys by heat treating them at a temperature of 225 C. and from the examples it will be'observed that the nature of the alloy determines the length of time required to reach the point of maximum growth. However, in every case the point of maximum growth is reached by a treatment extending for an appreciable time beyond that required for attaining maximum hardness, for otherwise maximum growth cannot be obtained;

According to the provisions of the atent statutes, we have. explained the principle and mode of operation of our invention, and have given several specific examples of the way in which it may be practiced. However, we desire to have it understood that, withinthe scope of the appended claims, the invention may be practiced otherwise than according to the specific examples, and that its beneficial results extend to aluminum base alloys which are subject to growth other than those an aluminum base alloy which is subject to and the alloy has attained a substantially growth, comprising heating the alloy to a complete and permanent increase in volume.

temperature of from about 150 to 265 C.

In testimonywhereof, we hereunto sign and continuing said heating until the hardour names.

ness of the alloy has passed through its maximum and the alloy has attained a substantially complete and permanent increase in volume:

"2. The method of develo ing substantially complete and permanent vo ume change in an aluminum base alloy containing copper and which is subject to growth when heated, comprising heatlng the alloy to a temperature of from about 150 to 265 C. and continuing said heating until the hardness of the alloy has passed through its maximum and the alloy has attained a substantially complete and permanent increase in volume.

3. The method of developing substantially complete and permanent volume change in an aluminum base alloy containing copper and silicon and WhlCh is sub ect to growth when heated, comprising heating the alloy to a temperature of from about 150 to 265 C. and continuing said heating until the hardness of the' alloy has passed through it maximum and the alloy has attained a substantially complete and permanent increase in volume.

4. The method of developing substantially complete and permanent volume change in an aluminum base alloy which is subject to growth, comprising heating the alloy to a temperature of from about 150 to 185 C.

- and continuing said heating until the hardness of the alloy has passed through its maximum and the alloy has attained a substantially complete and permanent increase in volume.

5. The method of developing substantially. complete and permanent volume change in a piston formed of an aluminum base alloy which is subject to growth, comprising heatin g the piston to a temperature of from about 150 to 265 C. and continuing said heating until the hardness of the alloy of which it is formed has passed through its maximum and the alloy has attained a substantially complete and permanent increase in volume.

6. An article of manufacture formed of an aluminum base alloy which is subject to having a Brinell hardness of not less than about and a structure obtained when the alloy of which it is formed is heated to a temperature of from about 150 to 265 C. until its hardness has passed through its maximum 

